Federal prosecutors have sent target letters to six Blackwater Worldwide security guards involved in a September shooting that left 17 Iraqi civilians dead, indicating a high likelihood the Justice Department will seek to indict at least some of the men, according to three sources close to the case.

The guards, all former U.S. military personnel, were working as security contractors for the State Department, assigned to protect U.S. diplomats and other non-military officials in Iraq. The shooting occurred when their convoy arrived at a busy square in central Baghdad and guards tried to stop traffic.

An Iraqi government investigation concluded that the security contractors fired without provocation. Blackwater has said its personnel acted in self-defense.

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The sources said that any charges against the guards would likely be brought under the Military Extraterritorial Jurisdiction Act, which has previously been used to prosecute only the cases referred to the Justice Department by the Defense Department for crimes committed by military personnel and contractors overseas. Legal experts have questioned whether contractors working for the State Department can be prosecuted under its provisions.

The sources cautioned that prosecutors are still weighing evidence gathered in a 10-month investigation that began shortly after the shootings. A federal grand jury has heard testimony from about three dozen witnesses since November, including U.S. and Blackwater officials and Iraqis, according to two of the sources.